The impact of water and pyridine on the α-iodination of α,β-unsaturated aldehydes and ketones

A direct α‑iodination of α,β‑unsaturated aldehydes and ketones carried out without metal bases at room temperature in water, utilizing 1–7 equivalents of pyridine, is reported herein. High yields are obtained in the iodination of enals. Liquid and solid enones give good to moderate yields of the iodo products. Pyridine was the most effective catalyst among the secondary and tertiary amines studied. An absence of water reduces yields, while yields of (Z)-α-iodo-α,β-unsaturated aldehydes and ketones improved significantly with water as the solvent. The reactivities of the enals and, to a certain extent, enones such as (E)-4-phenylbut-3-en-2-one and mesityl oxide could be accounted for by the Mayr electrophilicity parameter (E). However, the lower reactivities of (E)-hex-4-en-3-one and chalcone were the outliers of their E parameter values prediction.